The present invention relates generally to electrically connecting two electronic components, and, more particularly, to techniques for directly interconnecting copper pillars and copper bumps.
FIGS. 1A-1C are cross-sectional side views illustrating a conventional technique forming a die-substrate assembly 100 that includes a flip-chip integrated circuit (IC) die 110 having a copper (Cu) pillar bump 112 to a substrate 120 having a Cu pad 122. FIG. 1A shows the assembly 100 before a solder reflow step is performed, FIG. 1B illustrates the solder reflow step, and FIG. 1C shows the assembly 100 after the solder reflow step. Solder 130 is applied to one of the interconnect structures, in this case, the die's pillar bump 112, and flux 140 is applied to remove existing Cu oxide during the solder reflow step.
The solder reflow step involves uniform heating of the entire assembly 100 using, for example, conductive heating plates (not shown in FIG. 1B) and/or convective thermal wind to a temperature high enough to melt the solder 130. As shown in FIG. 1C, after cooling, the reflowed solder 130 forms both an electrical connection and a mechanical connection between the two Cu interconnect structures 112 and 122 to produce an inter-component joint 150.
It is known to use solder made of a metal, such as tin, having a melting point that is sufficiently low (e.g., below 300° C.) compared to the relatively high melting point (about 1085° C.) of the bulk copper used to form the interconnect structures 112 and 122 in order to prevent permanent damage to the die 110 and the substrate 120 from the heating applied during the solder reflow step.
Unfortunately, because the solder is made of a metal different from the copper of the interconnect structures 112 and 122, the resulting joint 150 can suffer from adverse intermetallic compound (IMC) effects that can affect the flow of current across the Cu—Sn interfaces of the joint 150. In addition, high current density can induce electro-migration (EM) where electrons can drive the Cu to diffuse into the solder, thereby increasing the adverse IMC effects.
Accordingly, it would be advantageous to have a way to interconnect such components where a connection joint therebetween does not suffer or suffers less from IMC and EM issues.